The present invention relates to an image processor used for image formation on a color printer which employs electronic photography.
In recent years, more and more color images have been printed by printers in offices and homes in line with widespread use of computers and color printers.
FIG. 12 illustrates the connection form of a host computer 50 and a color printer 1. The color printer 1 is connected to the host computer 50 via an interface 53 such as IEEE1284, or a network such as a LAN 51 and the internet 52. The color printer 1 and the host computer 50 communicate print data and printer status information with each other.
FIG. 13 is a block diagram of the printer. A numeral 2 designates a controller section for interpreting image data transmitted from the host computer 50 to generate a print image, and a numeral 3 designates a printer engine for forming print data on a recording medium by using the principle of electronic photography.
Configuration and operation of the controller are summarized below. The controller section includes an interface 4 for performing data communications with the host computer 50, an interpreter 5 for interpreting print data, a rasterizer 6 for forming print image on a memory, a compressor 7 for compressing print image, and an expander 8 for expanding compressed print image. Image data transmitted from the host computer 50 is input to the interpreter 5 via the interface 4. The interpreter 5 interprets the image data and creates drawing data. The rasterizer 6 extends print image on a band memory (not shown) where a single page is split in units of a plurality of lines base on drawing data. The extended print image is huge size so that it is temporarily compressed by the compressor 7 and saved into a compression memory (not shown). When a single-page print image is stored into the compression memory, operation of the printer engine 3 is started and the temporarily saved compressed print image is expanded by the expander 8 while being transmitted to the printer engine 3.
Next, configuration and operation of the printer engine 3 are described below.
The printer engine 3 includes a laser drive 9, a polygon mirror 10 which has undergone mirror finished in polygon, a photosensitive body 11 for forming an electronic latent image by way of a laser, developing units 12 for cyan, magenta, yellow and black (hereinafter referred to as C, M, Y and K), an intermediate transfer body 16 for transferring a toner image formed by the developing units 12 to retain a CMYK toner image, a paper cassette 17 accommodating recording paper, and a fuser 19 for fusing on paper with heat a toner image transferred onto paper. The laser drive 9 irradiates a laser light onto the polygon mirror 10 revolving in high speed while making laser blinking control in accordance with the data transmitted from the controller 2. A reflected laser light is irradiated onto the photosensitive body 11 and a latent image is formed on the photosensitive body 11. In this practice the main scan lines of the image are formed by the revolution of the polygon mirror 10. The latent image is formed by the developing unit 12 as a CMYK toner image. The toner image on each photosensitive body 11 is temporarily transferred onto the intermediate transfer body 16. The photosensitive body 11 and the developing unit 12 are serially arranged with respect to the drive direction of the intermediate transfer body 16 so that the intermediate transfer body can retain an image where CMYK toners on a single page have superimposed one on the other. The recording paper 13 is conveyed from the paper cassette 17 in synchronization with the movement of the intermediate transfer body. The toner image is transferred by a transfer unit 18 onto the recording paper 13 from the intermediate transfer body 16. Then the toner image is fixed with heat and a final output image is obtained.
FIG. 14 illustrates a related art image processor.
The image processor is included in the interpreter 5 in FIG. 13 and performs color conversion to device colors and binarization. The interpreter 5 includes a color converter 20 for converting RGB color signals to CMYK color signals, a gamma correction section 21 for correcting engine output characteristics, and a screen processor 22 for comparing an image with a threshold matrix and binarizing the image. Operation of each section is described below. RGB image data transmitted from the host computer 50 must be converted to CMYK data as device colors of the printer. The RGB image data is converted by a color converter 20. An RGB signal and a CMY signal comprise 256-level data having 0 to 255 levels, and in order to discriminate from the binary signal of CMYK, the signals are described as, for example R(255) in FIG. 14. In the color converter 20, the correspondence between an RGB signal and a CMYK signal has a non-linear characteristic. Thus, conversion from an RGB signal to a CMYK signal is made by retaining the correspondence between representative colors in a lookup table (hereinafter referred to as the LUT) and obtaining the points other than the representative points by interpolating the representative points. While the LUT is used to obtain the values of four colors CMYK in this example, it is possible to first obtain the values CMY in the LUT, then obtain CMYK through base color removal processing.
A CMYK signal as an output of the color converter 20 is further corrected by a gamma correction section 21. FIG. 15A shows an output density characteristic and FIG. 15B shows a gamma correction curve. In a printer engine using the principle of electronic photography, the relationship between an output signal and the density of an output image is linear as shown in FIG. 15A and differs depending on the material of a member such as a toner or for a printing process. This it is necessary to perform output level adjustment of CMYK independently of color conversion. By having gamma correction tables for CMYK representing the inverse function shown in FIG. 15B of the gamma correction curve in FIG. 15A and converting CMYK by using the tables, linearity of the output is obtained.
Next, binarization is made for each plane CMYK by the screen processor 22. FIG. 16 is an example of a screen matrix where threshold values corresponding to pixel levels of an image are arranged. By using separate threshold arrangements for four colors CMYK and setting a pixel equal to or larger than the threshold value of the screen matrix to 1 and setting a pixel smaller than the threshold value to 0, binary data of each of CMYK is obtained. In FIG. 14, description is made such as C(2) or CMYK(2) in order to represent CMYK binary data. Laser driving of the printer engine 3 is made based on the binary data.
By the way, there is known the Raster Operation processing (hereinafter referred to as the ROP processing) used for drawing objects on Windows widely used as the OS of the host computer 50. The ROP is a logical arithmetic processing made in superimposing more than one drawing object (raster image). The ROP processing performs a logical operation such as AND, OR, NOT and XOR on each object as well as an upper image of a set of an upper image and a lower image to give the effects of transparency and inversion. Note that the ROP processing is defined for an RGB image and C, M, Y, K data as device colors of a printer undergoes a logical operation by using the corresponding complementary colors. It is known that the ROP processing is unsuccessful in case this approach is used for binary data of CMYK thus it is necessary to use binary data of CMY without generating K data in color conversion (for example, refer to Takashi Hashizume, Kiyoshi Une, “Windows ni taiou shita PDL no iroshori-Raster Operation no taiou-”, Fuji Xerox technical report No. 12, 1998 tokushuu ronbun, [Retrieved Feb. 15, 2002] Internet URL: http://www.fujixerox.co.jp/randd/12/24 hasid/trl01j.html>).
FIG. 17 shows another example of a related art image processor. The image processor comprising a color converter for outputting CMY data performs RGB-to-CMY conversion in the color converter 20 then the ROP processing to generate a CMY print image. In case CMY binary data is transmitted to the printer engine without conversion and a CMY image is obtained as an output image, a black image appears as an image where three colors CMY are superimposed one on the other. This result is accompanied by numerous problems including: (1) The black color comprising three toner colors appears more chromatic; (2) A slight displacement in the print position of each color results in a black image surrounded by colors; (3) The total volume of toner is large so that transfer is unsuccessful in a printing process; (4) Fused toner comes off in scales; and (5) Toner three times as much as that required elsewhere is used, which is uneconomical.
In order to solve these problems, a simple binary CMY-to-binary CMYK conversion may be used whereby, of the binary CMY data, for the pixels in a portion where the CMY data corresponds to 1 at the same pixel position, K is replaced with 1 and CMY to 0.
FIG. 18 is a block diagram of a related art image processor where this processing is introduced. A numeral 20 designates a color converter for converting RGB color signals to CMY color signals, 21 a gamma correction section for correcting engine output characteristics, 22 a screen processor for comparing an image with a threshold matrix and binarizing the image, 24 an ROP processor for performing ROP processing, and 55 a K replacement processor for replacing CMY superimposed pixels with K.
FIGS. 19A to 19H illustrate the operation of the K replacement section. FIG. 19A shows a C plane, FIG. 19B an M plane, and FIG. 19C a Y plane. These figures show respective pixels in shading with lines. FIG. 19D shows three planes CMY superimposed one on the other, where the pixels in the center is the portion where CMY are superimposed. The pixels are replaced with K pixels. FIG. 19E shows a C plane obtained after K replacement, FIG. 19F an M plane obtained after K replacement, FIG. 19G a Y plane obtained after K replacement, and FIG. 19H a K plane obtained after K replacement.
In the printer engine using the principle of electronic photography, an attempt to form small dots shortens the laser lighting duration. This leads to insufficient latent image formation so that small dots are degenerated, shrink, or may disappear. As shown in FIGS. 19A through 19C, C and M are formed as concatenation of three pixels and Y as concatenation of two pixels before K placement. After K replacement, a shown in FIGS. 19E through 19H, all pixels are isolated pixels because pixels obliquely arranged have lower concatenation and assumed as isolated from each other. An image comprising CMY pixels before K replacement as shown in FIG. 19D turns into an image comprising isolated pixels so that all pixels disappear on the printed matter. In case this phenomenon is observed in a wide range, a portion where a CMY screen is superimposed in this way appears in a large cycle due to screen cyclicity, thus generating a cyclic dot dropout. An image to be reproduced smoothly as a monochrome appears as a texture. Even in case CMY not replaced remain concatenated with the surrounding pixels to avoid dot degeneration in the process the overlapping CMY pixels are replaced with K, a method to assume black color only at the position CMY pixels overlap tends to generate isolated pixels after K replacement, thus degeneration of isolated K pixels is inevitable. Gamma correction to correct engine characteristics is skipped on K generated on the overlapping CMY so that its reproducibility is considerably subject to the influence of printer engine characteristics. Another method is to set a small region on an ROP-processed binary CMY image to obtain the black density of the small region and further obtain a new CMYK pattern based on the black density (for example, refer to the Japanese Patent Laid-Open No. 2000-341547).
This method has problems: when for example a rectangular image in a color is on a white background, the black density differs between a case where a small region is provided inside the rectangle and a case where a small region is provided across a rectangular edge sections, so that the black density may not be detected correctly for the same color. It is necessary to set a large region to some extent in order to detect the black density. The black density in this region increases and the above problem will result, in case the region includes characters or a line drawing in black. The K pattern is not always generated on characters or a line drawing so that the characters or the line drawing will be degraded.